CO concentration and temperature sensor for combustion gases using quantum-cascade laser absorption near 4.7 μm

A sensor for sensitive in situ measurements of carbon monoxide and temperature in combustion gases has been developed using absorption transitions in the (v'=1a dagger vaEuro(3)=0) and (v'=2a dagger vaEuro(3)=1) fundamental bands of CO. Recent availability of mid-infrared quantum-cascade (QC) lasers provides convenient access to the CO fundamental band near 4.7 mu m, having approximately 10(4) and 10(2) times stronger absorption line-strengths compared to the overtone bands near 1.55 mu m and 2.3 mu m used previously to sense CO in combustion gases. Spectroscopic parameters of the selected transitions were determined via laboratory measurements in a shock tube over the 1100-2000 K range and also at room temperature. A single-laser absorption sensor was developed for accurate CO measurements in shock-heated gases by scanning the line pair vaEuro(3)=0, R(12) and vaEuro(3)=1, R(21) at 2.5 kHz. To capture the rapidly varying CO time-histories in chemical reactions, two different QC lasers were then used to probe the line-center absorbance of transitions vaEuro(3)=0, P(20) and vaEuro(3)=1, R(21) with a bandwidth of 1 MHz using fixed-wavelength direct absorption. The sensor was applied in successful shock tube measurements of temperature and CO time-histories during the pyrolysis and oxidation of methyl formate, illustrating the capability of this sensor for chemical kinetic studies.